Polycarbonates have been known for a number of years. U.S. Pat. No. 3,301,824 (1967) describes the preparation of carbonate homopolymers and random copolymers with lactones. The patent generally discloses the polymers as having utility in the molding, coating, fiber and plasticizer fields. There is no appreciation whatsoever of biodegradable devices composed in whole or in part of polycarbonate "biopolymers".
U.S. Pat. No. 4,243,775 (1981) and 4,429,080 (1984) disclose the use of polycarbonate-containing polymers in certain medical applications, especially sutures. However, this disclosure is clearly limited only to "AB" and "ABA" type block copolymers where only the "B" block contains poly(trimethylene carbonate) or a random copolymer of glycolide with trimethylene carbonate, and the "A" blocks are necessarily limited to polyglycolide. The dominant portion of the polymers is necessarily the glycolide component.
Accordingly, the art has failed to appreciate the potential biological or medical uses of block copolymers, having carbonates as their major component. This is especially true with respect to biodegradable or bioresorbable properties as well as the wide range of mechanical properties achieved with these materials for use in the fabrication of various devices.
Bioresorbable polymers have been used in the fabrication of devices for implantation in living tissue for several decades. Medical application of such polymers include absorbable sutures, haemostatic aids and, recently, intraosseous implants and slow-release drug delivery systems, to name but a few.
Use of such polymers has been extended to tissue regeneration devices such as nerve channels, vascular graft channels, sperm duct channels, fallopian tube ducts or channels and the like. To be effective, these devices must be made from materials that meet a wide range of biological and physical chemical prerequisites. The material must be bioresorbable at least in part, nontoxic, noncarcinogenic, nonantigenic, and must demonstrate favorable mechanical properties such as flexibility, suturability in some cases, and amenability to custom fabrication.
With particular emphasis on the replacement of injured, diseased, or nonfunctioning blood vessels, nonresorbable synthetic permanent vascular grafts have been available and are made of either Dacron (polyethylene terephthalate) or microporous Teflon (polytetrafluoroethylene). Various prostheses such as grafts, and especially those of small diameters for insertion in coronary bypass procedures, must have certain properties. These properties include physical and mechanical compatibility with the vessel to which they are connected, suturability, compliancy, ability to withstand pressure and pressure fluctuations, and flexibility. These properties also include biocompatibility--including such aspects as sterilizability and absence of toxicity, pyrogenicity, allergenicity, and mutagenicity; and adequate durability, both in terms of "shelf life" after fabrication and appropriate durability after implantation. Mechanical problems which can develop from the mismatch of a native vessel and a prostheses include elongation which results in, kinking, and perhaps in aneurysm formation and anastomotic hyperplasia. Vascular grafts of 8mm in internal diameter or larger, made of biodurable materials, have so far been the only successful prostheses, providing a conduit for maintaining continuous blood flow while inflicting a minimized and clinically tolerable hemotologic trauma. Most vascular grafts made of Dacron in current clinical use are of knitted or woven nonbiodegradable Dacron fibers with open pores in the fabric which have to be closed or diminished by preclotting before implantation. Such prostheses have been used as vascular replacements, but only for the relatively larger arteries. While bioresorbable materials have been proposed for use in such prostheses, the practical use of bioresorbable materials are, in general, currently limited to temporary devices such as fasteners, specifically sutures and pins.
Additionally, there has been an appreciation in the art of vascular prostheses, that macrophages infiltrate the implanted bioresorbable portion of the devices to digest bioresorbable materials, and aid in the formation of organized tissue which is comparable to that of the intact tissue. In particular, macrophages can be induced to secrete elastin in this tissue-reforming process. Induction of elastin secretion in the tissue repair can be accomplished by maintaining a mechanically dynamic environment as opposed to a static environment. While the desirability of maintaining a mechanically dynamic environment has been recognized, the art has not provided materials suitable for implantation which have this capability.